1. Field of the Invention
This invention relates to an extremely thin and higher strength microporous membrane of ultrahigh molecular weight polyethylene and a process for the production of the same. "Extremely thin" will hereinafter be referred to as "superthin" in this specification.
2. Description of the Prior Art
Porous materials have been used for various applications, e.g. separators for cells, diaphragms for electrolytic capacitors, various filters, moisture permeable and waterproof clothings, etc., but a thinner thickness and higher strength have lately been required therefor so as to miniaturize or weight-reduce these devices or to improve the performances thereof.
For the production of polyolefin porous materials, there have been proposed, for example, a method comprising mixing a polyolefin with a pore-forming agent consisting of a different polymer or a fine powder to prepare a microdispersion and extracting the pore-forming agent, a phase separation method comprising subjecting a polyolefin to a micro phase separation with a solvent to form a porous structure and a stretching method comprising forming a polyolefin in which different solids are microdispersed and stretching the formed polyolefin to give strain thereto, whereby the polyolefin and different solids are subjected to interfacial breakage to make micropores. However, these methods use polyolefins each having a molecular weight of less than 5.times.10.sup.5 and thus have a limit in attaining a superthin membrane with a higher strength by stretching. On the other hand, ultrahigh molecular weight polyolefins are expected to have a higher strength and higher elasticity even in the form of a superthin film, but in actual fact, they have respectively a worse stretchability because of more remarkable interlacement of the molecular chains than the ordinary molecular weight polyolefins. In order to improve this stretchability, for example, a method has been proposed comprising dissolving ultrahigh molecular weight polyethylene in a non-volatile solvent, forming a gel of fiber or film from this solution and then heating and stretching the gel containing the solvent or dry gel from which the solvent has been extracted with a volatile solvent, as described in Japanese Patent Application OPI (Kokai) No. 5228/1983. However, the gel having a porous network structure swelled highly with a non-volatile solvent has the drawbacks that when it is stretched in two directions, a higher orientation stretching is impossible, the network structure is enlarged to cause a tendency of breakage and the resulting film has a lower strength and larger diameter pores formed. When the gel is subjected to extraction of the non-volatile solvent with a volatile solvent and then dried, on the contrary, the network structure shrinks to be compacted, but the base stock sheet tends to bend by the non-uniform evaporation of the volatile solvent and a high ratio stretching cannot be effected due to the shrinking and compacting.